Asymmetric Digital Subscriber Line (ADSL) communications links are gaining in popularity and usage, usually for connection of an office or home computer to the Internet. ADSL communications equipment complies with a standard adopted by The Alliance For Telecommunications Industry Solutions (ATIS) in 1998. ATIS is a group accredited by the American National Standard Institute (ANSI). The ADSL standard used in North America is referred to as the ANSI T1.413 ADSL Standard, referred to herein as the T1.413 Standard, or, alternatively, the ADSL standard. As of the date of filing of this patent, essentially all new deployments of ADSL follow the ADSL standard. For more detailed information on the ADSL standard, see Palm (ed.), “G.dmt: asymmetrical digital subscriber line (ADSL) transceivers,” ITU-T Study Group 15 Question 4, G.992.1, 1999.
The ADSL standard was adopted to provide a way of transmitting data at high rates, up to ˜13 million bits per second, i.e., 13 Mbps, over twisted pair phone lines. The standard defines a discrete multi-tone (DMT) system that uses a number of “tones” or “sub-channels,” within a defined total channel bandwidth, that are each 4.3125 KHz wide for transmission of data between a telephone company central office (CO) and an end-user at customer premises.
Existing ADSL methods, including an enhanced ADSL standard referred to as ADSL2+ {for more information on ADSL2+, see F. van der Putten (ed.), “Draft recommendation G.adslplus (for consent),” ITU-T Study Group 15 Question 4, G992.5, 2003}, divide the available channel bandwidth into two passbands, one for the downstream direction, i.e., from CO to end-user, and one for the upstream direction, i.e., from end-user to CO. The division is such that the achievable data rate in the downstream direction is much greater than that in the upstream direction. This corresponds to the usage requirements of many homes and offices, as it is typically more common to download information than to upload information.
However, there is a large group of users in both home offices and small businesses for which it is desirable to have a bandwidth split which is not as biased towards higher data rates in the downstream. For these users, who also upload a significant amount of information, it is desirable to have a more symmetric upstream and downstream rate partition than currently possible with ADSL or ADSL+.
However, it is not possible to simply re-allocate tones from downstream to upstream, for example, to achieve this goal. This is because the ADSL standard defines a power spectral density (PSD) for communication, by specifying a PSD mask not to be violated. In the telephone infrastructure, the twisted pairs of many end-users in an area are eventually gathered together in a bundle that extends to the CO. The limits specified in the PSD mask prevent undue interference of ADSL signals on a given twisted pair with ADSL or other communications on other twisted pairs in the same bundle. Such interference can occur by having transmitted signal energy from one twisted pair couple to another twisted pair and contribute to the signals being received from that twisted pair, an effect referred to as cross-talk. If the cross-talk energy is sufficiently large, it can overwhelm a receiver, and actually prevent communication on the affected twisted pair. PSD is typically specified in units of dBm/Hz. As is well known in the art, a dBm is a unit of power in decibels, relative to one milliwatt.
The ADSL Standard sets forth the following limits: for the upstream direction the passband is 25-138 kHz. Within this band the power spectral density (PSD) is specified to be −38 dBm/Hz. The lower limit of the downstream passband depends upon the type of communication (frequency division multiplexing, or, FDM, versus echo cancellation), but in either case, above 138 kHz the PSD is specified to be −40 dBm/Hz.
It would therefore be desirable to have a method for enhancing ADSL upstream data rates that is compatible with existing services.